1. Field of the Invention
The present invention relates to a security mechanism for network communication, particularly to a method for key generation, member authentication and communication security in a dynamic group.
2. Description of the Related Art
The current cryptography systems may be categorized into the symmetric cryptography systems and the asymmetric cryptography systems. In the symmetric cryptography systems, the sender and receiver share the same key, and both parties use the same key to encrypt and decrypt the message. DES (Data Encryption Standard) is a symmetric cryptography system widely used in the early stage, but AES (Advanced Encryption Standard) has replaced DES now. The symmetric cryptography systems have the advantage of high speed. Further, the symmetric cryptography systems are hard to break if the key is long enough. However, the security of key administration may become a problem for the symmetric cryptography systems since many persons have the same key. In such a case, the symmetric cryptography systems need a complicated key administration system, and the keys should be updated periodically.
In the asymmetric cryptography systems, each user has a pair of keys, including a public key and a private key. The sender encrypts data with the public key, and the receiver decrypts data with the private key. The public key is widely issued to all the users. However, the private key is kept secretly. The asymmetric cryptography systems can provide privacy, authentication and undeniability at the same time and have a simpler key administration system. Each user only needs to keep his own private key no matter how many users are involved in the system. R. Rivest, A. Shamir and L. Adleman proposed an RSA public key cryptosystem, which is an asymmetric cryptography algorithm based on the difficulty of factoring a composite number into two large prime numbers. Among the public key cryptographies, the RSA public key cryptosystem is the most famous and widely-used. The Elliptic Curve Cryptography (ECC) is a new public-key cryptography algorithm based on the intractability of finding the discrete logarithm. As there is none algorithm able to find the discrete logarithm within an appropriate interval of time, the ECC-based cryptography is much securer than the RSA-based cryptography. Besides, the ECC-based cryptography can use a key of only 160 bits to achieve the same level of security afforded by the RSA-based cryptography using a key of 1024 bits. Therefore, the ECC-based cryptography is very suitable for the applications in a limited-resource environment, such as a smart card.
However, the asymmetric cryptography has the following disadvantages: complicated computation deceases encryption/decryption speed; the validity of a public key must be verified before using it; the data encrypted by a public key cannot be decrypted by the private keys of all the members but can only be decrypted by the private key of a single member; the public key of the opposite party must be verified before membership confirmation; the conventional asymmetric cryptosystems (such as RSA and ECC) are unlikely to protect privacy of members.
Accordingly, the present invention proposes a method for key generation, member authentication and communication security in a dynamic group to overcome the abovementioned problems. The principles and embodiments are to be described in detail below.